Hydraulic systems designed to move moveable components are for example used in vehicles such as cars and trucks. Taking the example of cars used for personal transport, such hydraulic systems are for example used for convertibles in which the convertible top can move from a closed position, in which the convertible top covers the passenger compartment of the vehicle, and an open position in which the passenger compartment is open. Similarly, such as system could also be used for opening and closing so-called tailgates, in particular for more expensive Sports Utility Vehicles (SUV's) and the like.
Such hydraulic systems have proven to be very convenient for such purposes as hydraulic system are capable of generating relatively much power even at small dimensions of such systems, which in turn makes it possible to use hydraulic systems even when space is limited.
A drawback of hydraulic systems when used in the mentioned applications often is the generation of sound that is considered disturbing by users. In particular when plunger pumps are used as a source for the hydraulic pressure, the pressure ripples in the hydraulic fluid that originate from the rotating plungers are a source of sound which is often in audible ranges. Furthermore, it is not uncommon that at least some of the components of the hydraulic system are, in particular when used in cars, mounted parts that amplify the sound caused by the pressure ripples in the hydraulic fluid. On top of that when using such systems for opening and closing tailgates or convertible tops it has been encountered that the frequency of the sound generated may vary quickly with the varying load on the hydraulic system. Furthermore, it is seen in such systems that the frequencies occurring in the hydraulic system are not limited to pump piston frequencies, but that also higher harmonic pulsations are present that may oscillate those parts in cars that have higher Eigenfrequencies. This will increase the level of noise. Hence, a range of frequencies may occur that consequently generate a range of noise.
Attempts have been made and solutions have been proposed to alleviate the above identified problems associated with such hydraulic systems.
U.S. Pat. No. 6,234,758 discloses a hydraulic system comprising a variable volume side branch for attenuating fluid noise. The side branch comprises a variable volume fluid container having an inlet and an outlet, wherein the variable volume container is a side branch located between the inlet and the outlet. Inside the container a displaceable piston is provided. Displacement of the piston in the container will change the volume thereof. The system further comprises a controller for controlling the position of the piston in response to the speed of the pump. Displacement of the piston in the container is effected by means of a geared motor and a threaded rod to which the piston is coupled. The variable volume container has a substantially T-shaped design having a main flow through tubular portion between the inlet and outlet intersected by the open end of the container. The container functions as a resonator, in which, by movement of the piston, the damping is tuned to the specific frequency generated by the pump. In fact, the length of the side branch or container is normally equal to a quarter of the wavelength of the frequency to be dampened so that a reflected pressure wave with a phase shift of 180 degrees is generated by the side branch that will cancel the pressure wave or pressure ripple. The resonator is identified in U.S. Pat. No. 6,234,758 as being of the Helmholz type.
A significant drawback of the system disclosed in U.S. Pat. No. 6,234,758 is that only one frequency can be dampened per position of the piston in the container. This is inherent to the functioning of such a Helmholz resonator. Although the piston can be displaced in the container and consequently different frequencies can in principle be dampened, the system is not able to dampen a range of frequency values which occur in hydraulic systems. Furthermore, the known system cannot react quickly to changes in the frequency to be dampened and requires a dedicated controller for adjusting the position of the piston in the container, which makes the system complex, expensive and unsuitable for dampening of ranges of frequencies which may change in magnitude.
In United States patent application US 2006/0243335 A1 a wave tuner is disclosed which is designed to cancel pressure pulses within a hydraulic system. The wave tuner is designed to reflect pressure pulses with a one hundred and eighty (180) degree phase shift to cancel the succeeding pressure pulse. Hence, the wave tuner of US 2006/0243335 A1 is tuned to one frequency, or a small band of frequencies around that frequency, only.
A further fluid borne noise suppressor for dampening fluid borne noise in a hydraulic system in known from U.S. Pat. No. 4,067,195. The suppressor comprises a cylindrical shaped chamber which is provided with cooling fins on its outer surface and acts as a cooling device for transferring heat from the hydraulic fluid to the surroundings.
A further pressure damper is known from U.S. Pat. No. 5,205,326 which damper comprises a housing in which an expandable bellows is provided. The expandable bellows is filled with N2 gas and can expand in the housing. The bellows can absorb pressure shock waves or pulsations and acts as a gas spring pulsation absorber.